There are many reasons why energy is wasted in connection with pump operation. In processing facilities and refineries, for example, priority is given to productivity and fast restarting following repairs. Often, this is achieved at the expense of a hydraulically optimised system. Frequently, when piping systems have been re-arranged for production-related reasons, the installed pump sizes are not adjusted to match. Oversized and undersized valves, inadequate pipe radii and encrusted pipe cross-sections throttle the flow and contribute to energy waste. Also, pumps are often left to run far off their originally envisaged operating point and many are selected with a 10 or 15% safety margin.

For new installations, it is relatively easy to select a hydraulically optimised pump, as long as the requirements to be met by the pump are known. It is important that a large number of closely spaced pump sizes are available from the manufacturer and that impeller diameters are trimmed to match the required operating point. It is also important that no safety margins are added; even efficient pumps cannot run economically if they are used in the wrong operating mode.

When a system has been in service for some years, it may not be all that easy to determine its actual savings potential. The first step is to ascertain the pump’s present operating point and mode of operation; all other cost-cutting measures depend on them. Professional assistance may prove useful. KSB, for example, offers what it calls ‘SES’ (System Efficiency Service). A site survey is conducted; data loggers measure flow rates, pump suction and discharge pressures and electrical data such as the momentary motor outputs. The pump’s bearing bracket vibrations are also measured. 

 

This gives information on the wear state of each pump, enabling identification of misalignments, warping, unbalance and vibrations from the periphery. KSB engineers use the data to suggest cost-cutting measures and to calculate anticipated payback. If one of the measures is implemented, subsequent measurements are performed to show the measure of success.

Use of variable speed pump sets suggests itself for pump systems with time-dependent flow rates. What is perfectly normal in other fields of application, i.e. matching power input to actual demand, is still far from standard practice in many branches of industry. 

Detailed analysis of a pump’s operating behaviour often reveals that it is not running at its optimal, energy-efficient operating point. A common cause is that it is operating under off-design conditions due either to system oversizing or to normal temporary variations in flow rate arising from system processes. If power input is not adjusted to demand via some form of system control, energy is wasted. The pump’s power input can be matched to the precise requirements of the system by altering its speed. In the case of closed-circuit systems, energy savings of up to 60% can be achieved depending on load profile.

By fitting a speed control unit a simple differential pressure keeps the discharge pressure constant, which can produce encouraging energy cost savings. Such savings can be achieved with almost any standard frequency inverter. If the hydraulic function ‘compensation of flow-dependent pipe friction losses’ is added – a feature offered by KSB’s PumpDrive variable speed system, for example – the savings can increase almost four-fold compared with the fixed speed version. 

 

Potential savings are dependent on the system’s load profile. The most significant cost reductions are achievable when a pump frequently operates under low flow conditions. Speed control will not save costs on pumps whose total volume flow rate is required continuously.

The range of off-the-shelf variable speed systems for use in potentially explosive atmospheres remains small. Indeed, the operators’ myriad different works standards make it difficult for pump manufacturers to economically accommodate their ‘motor-mounted’ variable speed systems to the respective set of requirements and then get them certified. The same applies to new high-efficiency drive motors. While they do generate less heat and would therefore be easy to adapt for such applications, their working principle often makes them dependent on an accessory frequency inverter.

Modern centrifugal pumps achieve a high level of efficiency. When they appear to be consuming more energy than necessary, it is probably because people pay too little attention to pumps that just keep running. No pump can operate more economically than its surrounding system allows.